This invention relates to Tactical Air Navigation (TACAN) in general and in particular to digital TACAN systems. Still more particularly, this invention relates to methods and apparatus for converting digital TACAN output signals into an analog potentiometer signal suitable for utilization with an analog navigation computer.
TACAN is a short range navigation system which can be utilized to provide continuous and accurate distance and bearing information. This information is generally visually presented utilizing a display instrument on an aircraft instrument panel. An integral part of the TACAN system is Distance Measuring Equipment (DME) which is utilized to calculate the range to the TACAN station or ground beacon.
DME is utilized to determine the distance or range to the TACAN ground beacon by measurement of the elapsed time between transmission of an interrogating pulse by an airborne TACAN set and reception of corresponding reply pulses from the ground beacon. The transmitter in the aircraft begins the interrogation process by sending out interrogation pulses at a low pulse repetition frequency. The ground beacon then triggers its transmitter which sends out the reply pulse signals. These signals require about twelve microseconds round trip travel time per nautical mile of distance from the ground beacon. A range indicator is then utilized to generate a visual indication of range to the beacon.
The range ouptut of a TACAN set is also commonly utilized in conjunction with a navigation computer to generate solutions to navigational problems. A problem which has recently arisen in this area occurs due to the fact that many manufacturers are now building TACAN sets which utilize digital output signals and many existing navigation computers are analog devices. That is, the range input required by these analog devices is a potentiometer type input. A simple digital to analog conversion is not feasible in these cases due to the nature of a potentiometer which varies the resistance of a single point while maintaining a constant voltage drop across the entire device.
It should therefore be apparent that a need exists for a method and apparatus for converting the digital range output of a TACAN set to an analog potentiometer type input for utilization with an analog navigation computer.